In gas turbine engines, one or more rotors of the engine can be subjected to axial thrust loads during operation. Thrust loads arise in a gas turbine engine as the result of pressure imbalances. For example, a compressor has a higher downstream pressure than upstream pressure which forces the compressor upstream (towards the intake) whereas a turbine has a higher upstream pressure than downstream pressure which forces the turbine downstream (towards the exhaust nozzle). The thrust loads urging the compressor upstream and the turbine downstream are high and uncertain.
The thrust loads are often at theft maximum during the periods of highest power output for the engine. In a gas turbine engine providing jet propulsion for an aircraft, this period of maximized power output can occur when the aircraft is taking-off and/or climbing to a cruising altitude. The thrust loads can change direction (passing through a zero bad point) during a flight cycle.
A thrust bearing can be positioned to support the rotor against these thrust loads. A thrust bearing typically comprises an inner and outer race, a cage and a set of rolling elements, the rolling elements being spheres (or balls) which are contained within a raceway formed in one or both of the races with the cage maintaining the spacing between the balls.
Since single thrust bearings inevitably have a limited thrust capability, two or more bearings may be arranged adjacent one another to share the thrust load. In these bearing arrangements, under-loading of one set of rolling elements may result in “skidding” of that set, which may cause damage, debris release and bearing failure. There is a desire to provide a bearing arrangement which improves the balancing of thrust load bearing between bearings in order to reduce excessive loading or under-loading of a bearing thus reducing bearing failure and vibration/skidding.